The present invention relates to a process for catalytically converting a naphtha containing olefin in a process using a shape selective catalyst that does not require steaming to provide activity and selectively. More particularly, the invention relates to the use of such catalysts for producing light (i.e., C2-C4) olefins from a naphtha, and preferably from a catalytically cracked or thermally cracked naphtha stream. The naphtha stream is contacted with a catalyst containing from about 10 to 50 wt. % of a crystalline zeolite having an average pore diameter less than about 0.7 nanometers at reaction conditions which include temperatures from about 500xc2x0 C. to about 650xc2x0 C. and a hydrocarbon partial pressure from about 10 to 40 psia.
The need for low emissions fuels has created an increased demand for light olefins for use in alkylation, oligomerization, MTBE and ETBE synthesis processes. In addition, a low cost supply of light olefins, particularly propylene, continues to be in demand to serve as feedstock for polyolefin, particularly polypropylene, production.
Fixed bed processes for light paraffin dehydrogenation have recently attracted renewed interest for increasing light olefin production. However, these types of processes typically require relatively large capital investments as well as high operating costs. It is, therefore, advantageous to increase light olefin yield using processes which require relatively small capital investment. It would be particularly advantageous to increase light olefin yield in catalytic cracking processes.
U.S. Pat. No. 4,830,728 discloses a fluid catalytic cracking (FCC) unit that is operated to maximize light olefin production. The FCC unit has two separate risers into which a different feed stream is introduced. The operation of the risers is designed so that a suitable catalyst will act to convert a heavy gas oil in one riser and another suitable catalyst will act to crack a lighter olefin/naphtha feed in the other riser. Conditions within the heavy gas oil riser can be modified to maximize either gasoline or light olefin production. The primary means of maximizing production of the desired product is by using a specified catalyst.
Also, U.S. Pat. No. 5,026,936 to Arco teaches a process for the preparation of propylene from C4 or higher feeds by a combination of cracking and metathesis wherein the higher hydrocarbon is cracked to form ethylene and propylene and at least a portion of the ethylene is metathesized to propylene. See also, U.S. Pat. Nos. 5,026,935;5,171,921 and 5,043,522.
U. S. Pat. No. 5,069,776 teaches a process for the conversion of a hydrocarbonaceous feedstock by contacting the feedstock with a moving bed of a zeolitic catalyst comprising a zeolite with a pore diameter of 0.3 to 0.7 nm, at a temperature above about 500xc2x0 C. and at a residence time less than about 10 seconds. Light olefins are produced with relatively little saturated gaseous hydrocarbons being formed. Also, U.S. Pat. No. 3,928,172 to Mobil teaches a process for converting hydrocarbonaceous feedstocks wherein light olefins are produced by reacting said feedstock in the presence of a ZSM-5 catalyst.
One problem inherent in conventional light olefin production using FCC units is that the process depends on a specific catalyst balance to maximize production of light olefins while also achieving high conversion of the 650xc2x0 F. plus feed components. In addition, even if a specific catalyst balance can be maintained to maximize overall light olefin production, light olefin selectivity is generally low due to undesirable side reactions, such as extensive cracking, isomerization, aromatization and hydrogen transfer reactions. Light saturated gases produced from undesirable side reactions result in increased costs to recover the desirable light olefins. Therefore, it is desirable to maximize olefin production in a process that allows a high degree of control over the selectivity to light olefins.
Another problem associated with conventional olefin production via the cracking of higher molecular weight hydrocarbon species using zeolite catalysts is that the catalyst requires steam activation prior to use to provide sufficient conversion activity. Moreover, some conventional light olefin processes using catalyst steam activation exhibit little if any light olefin selectivity increase in connection with the activity increase. The catalyst may be activated prior to use in a light olefin conversion reaction, thereby increasing process and equipment requirements. Alternatively, it may be activated during the light olefin conversion reaction by adding steam to the feed. This method detrimentally reduces initial light olefin yield compared to steady state yield because the initial catalyst charge requires a period of time for activation. In-situ steam activation also leads to a diminished steady-state yield because fresh catalyst make-up added during the process requires a period of time for activation. There is, therefore, a need for a catalyst that does not require steam activation to selectively produce light olefins from a catalytically or thermally cracked naphtha containing paraffins and olefins.
The invention relates to a catalytic conversion process comprising:
contacting a naphtha containing olefins with a catalytically effective amount of a catalyst, wherein the catalyst contains 10 to 80 wt. % of a molecular sieve having an average pore diameter less than about 0.7 nm, under catalytic conversion conditions in order to form a product, wherein the catalyst""s Steam Activation Index is greater than 0.75.
The invention also relates to a catalytic conversion process, comprising:
contacting a naphtha containing olefins with a catalytically effective amount of a molecular sieve catalyst under catalytic conversion conditions in order to form a product containing propylene, wherein
(a) the molecular sieve catalyst contains 10 to 80 wt. % of a crystalline zeolite, based on the weight of the catalyst, having an average pore diameter less than about 0.7 nm;
(b) the molecular sieve catalyst contacts steam
(i) at a steam pressure in a steam pressure range of from 0 atmospheres to about 5 atmospheres prior to catalytic conversion,
(ii) with a steam amount in a steam amount range of from 0 mol. % to 50 mol. %, based on the amount of the naphtha, during catalytic conversion, and
(iii) during a combination of (i) and (ii); and
(c) the weight ratio of the propylene in the product to the naphtha changes by less than about 40% over the steam pressure range, the steam amount range, and combinations of the steam pressure range and steam amount range.
In yet another embodiment, the invention relates to a catalytic conversion process, comprising:
contacting a naphtha containing olefins with a catalytically effective amount of a molecular sieve catalyst under catalytic conversion conditions in order to form a product containing propylene, wherein the molecular sieve catalyst contains 10 to 80 wt. % of a crystalline zeolite having an average pore diameter less than about 0.7 nm, with the proviso that if the molecular sieve catalyst contacts steam
(i) at a steam pressure ranging from 0 atmospheres to about 5 atmospheres prior to catalytic conversion,
(ii) at a steam amount ranging from 0 mol. % to 50 mol. %, based on the amount of the naphtha, during the catalytic conversion, and
(iii) during a combination of (i) and (ii), then the catalyst""s catalytic activity for forming the propylene is substantially insensitive to the steam amount, the steam pressure, and combinations thereof.
In a preferred embodiment the invention is a process for selectively producing light olefins in a process unit comprised of a reaction zone, a stripping zone, and a catalyst regeneration zone. The naphtha stream is contacted in the reaction zone, which contains a bed of catalyst, preferably in the fluidized state. The catalyst is comprised of a zeolite having an average pore diameter of less than about 0.7 nm. The reaction zone is operated conventionally at a temperature from about 525xc2x0 C. to about 650xc2x0 C., a hydrocarbon partial pressure of 10 to 40 psia, a hydrocarbon residence time of 1 to 10 seconds, and a catalyst to feed weight ratio of about 2 to 10.
In another preferred embodiment of the present invention the molecular sieve catalyst is a zeolite catalyst, more preferably a ZSM-5 type catalyst.
In still another preferred embodiment of the present invention the feedstock contains about 10 to 30 wt. %. paraffins, and from about 20 to 70 wt. % olefins, and no more than about 20 wt. % of the paraffins are converted to light olefins.
In yet another preferred embodiment of the present invention the reaction zone is operated at a temperature from about 525xc2x0 C. to about 650xc2x0 C., more preferably from about 550xc2x0 C. to about 600xc2x0 C.